To make your Google Calendar public, on a computer, open Google Calendar.
  1. In the top right, click Settings Settings (Gear Icon) > Settings.
  2. Open the Calendars tab.
  3. Click the name of the calendar you want to share.
  4. Open the Share this Calendar tab.
  5. Check the option Make this calendar public. 
  6. If don't want other people to view the details of your events, select See only free/busy (hide details).
  7. Click Save.
Google Apps users: Your domain's administrator may restrict your sharing settings. If you you don't see some sharing settings, contact your domain administrator.

Here is a link from Google on how to change the Calendar to Public https://support.google.com/calendar/answer/37083?hl=en&vid=0-40489796414-1479845483102&visit_id=1-636154423320968037-2143610516&rd=1.